Various types of wireless communication systems involve signals from multiple users that are processed at a central location. In such systems, a basestation may handle traffic simultaneously from many different transceivers (terminals) that are operated by a multitude of different users. Such systems are often organized into geographic subdivisions, such as sectors so that the different signals can be effectively combined at a single basestation. A common example of such as system is a cellular phone network, in which a cellular basestation located in the center of a cell handles phone traffic for a number of users at the same time. In order to increase capacity, cells may be further subdivided into a number of sectors through sectorization techniques such as focused antenna arrays and/or time or frequency multiplexing methods. The communication paths in such wireless systems are usually separated into downlink (basestation-to-terminal) communications and uplink (terminal-to-basestation) communications.
In wireless communication systems, the quality of the signal depends in large part on the amount of noise measured at the receiver antenna of both the basestation and the terminals. In general, there are three main sources of noise at a receiver antenna: (1) ambient (non-system) noise or interference; (2) circuit noise, caused by the circuits of the receiver itself; and (3) system interference, which is introduced by transmitters or sources other than the target transceiver. The signal-to-interference-plus noise ratio (SINR) is a measure of the quality of the signal received by a terminal against the noise and interference in the system, and a higher SINR value indicates a higher quality of transmission in a wireless link. Thus, the amount of noise and system interference in the signal, as measured by the SINR value, determines the quality of the transmission link and generally dictates how much data can be carried on the communication line. The SINR level may differ depending on the location of a receiver within a cell or sector of the wireless system, and can also vary depending upon the composition or amount of noise versus interference, or vice-versa.
Users (terminals) in the uplink portion of a cellular, or similar system that are on or near the boundaries between cells or sectors usually have low SINR values because of the potentially strong interference from terminals of neighboring cells or sectors, or the large distance of the terminal from the basestation. As a result, these boundary-terminals often suffer from poor transmission quality, and may be forced to transmit at lower communication rates or move to more optimum locations within the cell or sector.
This effect is particularly pronounced in wireless transmission systems that utilize OFDM (Orthogonal Frequency Division Modulation) schemes or similar cellular systems, as the base stations are configured to simultaneously process communication traffic from multiple users at any one time. A multi-user version of OFDM is OFDMA (Orthogonal Frequency Division Multiple Access), which assigns subsets of subcarriers to individual users, thus allowing simultaneous transmission from several users. OFDMA systems may employ a “frequency reuse-one” technique, in which every cell and sector is free to utilize all of the subcarriers and symbols used in other cells and sectors. Such a system can have significant interference between sectors and cells, especially at the boundaries. Present interference mitigation techniques are generally ineffective at eliminating interference in terminals that are located at cell or sector boundaries. Such techniques may employ filtering or similar techniques to reduce interference, however, most present systems do not employ any coordination across sector or cell boundaries in order to improve uplink transmission quality between target terminals and basestations within each sector. Such coordination can often be advantageous in that it can allocate specific resources, such as power and time/frequency slots, that may be better utilized by a target terminal rather than an interfering terminal.